Active eyewear (sometimes referred to as 3D active eyewear, liquid crystal shutter (LCS) glasses, etc.) is eyewear used in conjunction with a display screen (e.g. of a television or computer monitor) to create the illusion of a three dimension image. The right and left lenses of active eyewear are controlled separately to alternate between being transparent (“open”) and dark (“closed”). With LCS glasses, each lens includes a liquid crystal layer (“shutter”) that is normally transparent but which becomes dark when a voltage from a shutter switching signal is applied.
Active eyewear can be controlled to alternately darken lenses in synchronization with the frame refresh rate of a screen which alternates between frame images taken from different perspectives. This technique is referred to as alternate-frame sequencing, which achieves the desired stereoscopic effect by having each eye see only the image that was intended for it. The synchronization between the active eyewear and the display screen is often achieved wirelessly, e.g. via infrared (I/R) transmission from an I/R transmitter associated with the display screen to an I/R receiver of the active eyewear.
FIG. 1 illustrates an alternate-frame sequencing system 10 in accordance with the prior art. A display screen 12, such as of a 3D television set (3D-TV) 14, has an I/R transmitter 16 which transmits an I/R synchronization signal 18 in response to a SYNC signal generated by the 3D-TV. Active eyewear 20, having a left lens 22 and a right lens 24, is provided with one or more I/R photodiodes 26 that are sensitive to the I/R synchronization signal 18. The photodiodes 26 are also sensitive to I/R interference 28 created by, for example, sunlight 30, incandescent light 32 and fluorescent light 34.
FIG. 2 illustrates an example I/R synchronization signal of the prior art. In this example, the I/R synchronization signal includes a series of encoded commands such as OPEN LEFT, CLOSE LEFT, OPEN RIGHT, CLOSE RIGHT, referring to the opening and closing of the left and right lenses, respectively, of the active eyewear 20. These commands are generally modulated to provide one or more pulses which encode the commands, e.g. the OPEN LEFT command 36 could include one pulse, the CLOSE LEFT command 38 could include two pulses, OPEN RIGHT command 40 could include three pulses, and the CLOSE RIGHT command 42 could include four pulses. By way of example, the pulse width of the base band signal may be a few hundred microseconds, and it may be chopped with a square wave carrier having a chop rate that is, for example, a about an order of magnitude less.
It should be noted the illustration of FIG. 2 is not to scale. For example, the duty cycle of the commands is generally 10% or less than the command cycle.
Unfortunately, there is not currently a generally accepted standard format for I/R synchronization signals 18. Therefore, while the I/R synchronization signal of FIG. 2 can be used to generically describe I/R synchronization signals of the prior art, it will be appreciated that different manufacturers may have differ timing between commands, send commands at different rates, or may encode the commands differently.
Typically, the active eyewear circuitry for controlling the lenses 22 and 24 is provided in the frame of the active eyewear 20. Also provided in the frame would be the one or more I/R photodiodes 26, a power supply (e.g. batteries), and perhaps an on/off switch.
The active eyewear circuitry of the prior art typically uses a transimpedance amplifier (TIA) as a low-noise front-end circuit to convert the current signal from the I/R photodiode(s) 26 to a voltage signal. Negative feedback of the TIA lowers the input impedance to increase bandwidth in compensation for the inherently large capacitances of photodiodes. A subsequent voltage-to-current converter is then typically needed convert the output voltage of the TIA to a current that can be input into a variable gain current amplifier.
It will be appreciated that the TIA system is relatively complex, utilizes a number of components, and consumes a considerable amount of power. This can be problematical in that the active eyewear circuitry is typically powered by small batteries provided in the frames of the active eyewear 20 and the high current drain of the TIA and associated circuitry will reduce the amount of time that the active eyewear 20 can be used before the batteries need to be recharged or replaced.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.